Many fields today require weight-optimized products without sacrificing function or strength. This is particularly true of forged products, which can be heavy and difficult to optimize due to limitations of the tools used for manufacture.
One example is front axle beams for heavy vehicles. These beams are typically forged as an I-profile, where the web or core in the beam cross-section has very little effect on the torsional rigidity. With strength calculations it can be shown that a tubular cross-section, with a material moved radially outwards as far as possible, is optimal for such a structure. This is particularly true of the so-called “swan neck” on the front axle beam, between its central portion and a king pin support. With conventional forging technology it is, however, difficult to achieve this. EP-A2-0 059 038 shows a front axle beam forged lying in a conventional manner, i.e. the blank lies with its final vertical plane (after mounting) in the horizontal plane during working. The specification describes how a blank is pre-shaped by means of rolling and is then moved between a number of presses, which forge the entire blank or portions thereof to the desired shape. The disadvantage of this method is, as was described above, that the web of the beam is largely located centrally, which has very little effect on the torsional rigidity.
An alternative solution is shown in EP-A1-0 015 648, which describes the forging of a rectangular hollow front axle beam, starting from a tubular blank. While it is true that it is possible to obtain a beam with higher torsional rigidity with this method, it involves a number of problems. To produce the tapered ends of the beam these must be pulled through a die. Even if the material is distributed radially further out from the centre of the beam, the possibility of controlling the thickness of the material is very limited. This also applies to the other parts of the beam, since the starting material is a tube with constant thickness. Quite some work is required on the ends of the beam to provide king pin supports and the mounting of separate mounts for air bellows, for example.
Another solution is revealed in U.S. Pat. No. 6,122,948, which shows a hydroformed front axle beam. In this case as well one starts from a tubular blank, which is first bent to the desired basic shape and is then hydroformed to its final shape. The disadvantage of this solution is firstly that, as in the example above, it is not possible to control the distribution of material along the length of the profile. One must also provide the profile with a number of separate mounts, not only for the air bellows but also for the king pins. The latter must be welded on, for example, which provides the beam with a natural weak point susceptible to corrosion.
Finally, it is also possible to cast hollow front axle beams as is shown in JP-A-11-011105. For reasons of casting technology, there are, however, limits as to the greatest and smallest thickness and requirements for reinforcing ribs, complicated casting cores and the like, to permit casting of such an advanced profile. Beyond this, there are additional limits as regards which materials are practically possible and the economic consequences on the piece price of the axle beams due to the great increase in costs which a casting process would involve.
Most of the above mentioned problems can be solved by the manufacturing method according to the invention. This method makes it much more possible to achieve exact control of the distribution of material around and along a forged profile.